Update licensing information after repository refactoring

In order to disambiguate things now that libm is part of the
compiler-builtins repository, do the following:

* Mention libm in LICENSE.txt
* Clarify the default license for crates other than libm and
  compiler-builtins
* Add an explicit license field to Cargo.toml for all other crates

library/compiler-builtins/LICENSE.txt

@@ -1,10 +1,14 @@
-compiler-builtins as a whole is available for use under both the MIT license
+The compiler-builtins crate is available for use under both the MIT license
 and the Apache-2.0 license with the LLVM exception (MIT AND Apache-2.0 WITH
 LLVM-exception).
 
-As a contributor, you agree that your code can be used under either the MIT
-license, or the Apache-2.0 license, or the Apache-2.0 license with the LLVM
-exception.
+The libm crate is available for use under the MIT license.
+
+As a contributor, you agree that your code may be used under any of the
+following: the MIT license, the Apache-2.0 license, or the Apache-2.0 license
+with the LLVM exception. In other words, original (non-derivative) work is
+licensed under MIT OR Apache-2.0 OR Apache-2.0 WITH LLVM-exception. This is
+the default license for all other source in this repository.
 
 Text of the relevant licenses is provided below:
 
@@ -263,8 +267,9 @@ license:
 * Work derived from compiler-rt after 2019-01-19 is usable under the
   Apache-2.0 license with the LLVM exception.
 
-* The bundled `math` module is from rust-lang/libm, usable under the MIT
-  license. See https://github.com/rust-lang/libm for details.
+* The bundled `math` module is from the libm crate, usable under the MIT
+  license. For further details and copyrights, see see libm/LICENSE.txt at
+  https://github.com/rust-lang/compiler-builtins.
 
 Additionally, some source files may contain comments with specific copyrights
 or licenses.

library/compiler-builtins/builtins-test-intrinsics/Cargo.toml

@@ -3,6 +3,7 @@ name = "builtins-test-intrinsics"
 version = "0.1.0"
 edition = "2021"
 publish = false
+license = "MIT OR Apache-2.0"
 
 [dependencies]
 compiler_builtins = { path = "../compiler-builtins", features = ["compiler-builtins"]}

library/compiler-builtins/builtins-test/Cargo.toml

@@ -4,6 +4,7 @@ version = "0.1.0"
 authors = ["Alex Crichton <alex@alexcrichton.com>"]
 edition = "2024"
 publish = false
+license = "MIT AND Apache-2.0 WITH LLVM-exception AND (MIT OR Apache-2.0)"
 
 [dependencies]
 # For fuzzing tests we want a deterministic seedable RNG. We also eliminate potential

library/compiler-builtins/crates/libm-macros/Cargo.toml

@@ -3,6 +3,7 @@ name = "libm-macros"
 version = "0.1.0"
 edition = "2024"
 publish = false
+license = "MIT OR Apache-2.0"
 
 [lib]
 proc-macro = true

library/compiler-builtins/crates/musl-math-sys/Cargo.toml

@@ -3,6 +3,7 @@ name = "musl-math-sys"
 version = "0.1.0"
 edition = "2024"
 publish = false
+license = "MIT OR Apache-2.0"
 
 [dependencies]
 

library/compiler-builtins/crates/util/Cargo.toml

@@ -3,6 +3,7 @@ name = "util"
 version = "0.1.0"
 edition = "2024"
 publish = false
+license = "MIT OR Apache-2.0"
 
 [features]
 default = ["build-musl", "build-mpfr", "unstable-float"]

library/compiler-builtins/libm-test/Cargo.toml

@@ -3,6 +3,7 @@ name = "libm-test"
 version = "0.1.0"
 edition = "2024"
 publish = false
+license = "MIT OR Apache-2.0"
 
 [features]
 default = ["build-mpfr", "build-musl", "unstable-float"]